Call plug_vifs() for all instances in init_host

Remove the assumption that we are using the "flat" networking model
and there is a single bridge interface for all instances on a given
compute node.  Instead simply call plug_vifs() for each instance in
init_host.

Fixes bug 1083784

Cherry-picked from 77e38b7cfc2206ef4ac528d3c9faaa09adbf7f58

Issue: altai-148
Change-Id: I66eecfb156f20118b9f6a75b1d8b97613f69a349